Magnetic measuring device and method



Aug. 9, 1949. R. DE 0. M DILL MAGNETIC MEASURING DEVICE AND METHOD 3Sheets-Sheet 1 Filed June 21, 1943 Aug. 9, 1949. R. DE 0. M DILL2,473,773

MAGNETIC MEASURING DEVICE AND METHOD Filed June 21, 1943 3 Sheets-Sheet2 fig- 3.

[III/III).

2.27m? REX DE ORE M /LL.

Aug. 9, 1949. R. DE 0. McDlLL MAGNETIC MEASURING DEVICE AND METHOD 3Sheets-Sheet 5 iled June 21, 1945 .ZZE

valve.

Patented Aug. 9, 1949 MAGNETIC MEASURING DEVICE AND METHOD Rex De OreMcDill, Cleveland, Ohio, assignor to Thompson Products, Inc., acorporation of Ohio Application June 21, 1943, .Serial No. 491,664

Claims. (Cl. 175-183) This invention relates to a measuring device.

and more particularly to a method and means for measuring wallthickness, permeability and resistivity of ferro-magnetic structures.

In the past it has been dimcult to make the above measurements with anygreat degree of accuracy where one side of the wall has beeninaccessible and where the instrumentalities for carrying out themeasurements were necessarily all located on one side thereof.

One or the principal features and objects of this invention is toprovide a novel method and means for measuring wall thickness,permeability and resistivity by an eddy current penetration and magneticflux method.

A more specific feature and object of the present invention is toprovide a novel method and means for measuring the wall thickness in thehead and skirt of an austenitic steel aircraft or other type valve whichis partially filled with sodium.

Another object of the present invention is to provide a novel method andmeans for accurately measuring the sodium content of a sodium filled Afurther object of the present invention is to provide a measuringinstrumentality which in cludes a coil carrying an audio-frequencycurrent which is brough into proximity with a ferromagnetic structure,and the change in the inductance of the coil noted.

A still further object of the present invention is to provide a novelcircuit arrangement for measuring apparatus.

The novel features which I believe to be characteristic of my inventionare set forth with particularlty in the appended claims. My inventionitself, however, both as to its manner of construct-ion and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description, taken inconnection with the accompanying drawings, in which:

Figure 1 is a wiring diagram of one embodiment of the present invention;

Figure 2 is a partial sectional view of an aircraft valve in associationwith the detector coil of the measuring device and of a supportingstructure'ior holding the valve directly above the detector coil;

Figure 3 is a plan view of the detector coil and the supportingstructure of Figure 2 if the valve were moved;

Figure 4 is a diagrammatic representation of the detector coil in closeassociation with a ferromagnetic plate. which plate is backed up byanother metal and Figure 5 is a circuit diagram of a differentembodiment of the present invention.

In Figure 1 of the drawings a wiring diagram is shown which illustratesone embodiment of the present invention. As diagrammatically shown, thecircuit includes a detector coil, abridge, a sweep oscillator havingclosely regulated voltage and frequency, and an audio-amplifier, theoscillator and amplifier ha over a wide range of frequencies. 7

Referring now'to Figure 1, power is supplied to the circuit through avoltage regulating transformer ill, a power transformer II and a fullwave rectifier tube ll. Three secondary windings, I 3, H and I! areprovided on transformer ll. Secondary winding I3 is thefilament heatingcurrent winding for the various electron discharge devices provided inthe circuit, and although the complete circuit is not shown, it will beunderstood from the diagrammatic representationjthat-th'e filament ofeach of the electron discharge devices presently to be described, withthe exception of tube It, is connected to winding it and obtains a lowvoltage alternating current therefrom.

Secondary winding i4 isconnectedto the filament or directly heatedcathode IQ of rectifier tube i2. Secondary winding II has its oppositeends connected to anode plates I1 and I8 of rectifier tube It and hasits midpoint grounded as at l9.

- A sweep oscillator is provided by gas triode 20, charging resistors IIand 22 and condensers 23 and 24, which condensers arearranged to beselectively connected between the cathode 25 of gas triode 20, and plate2. thereof through plate resistor 21. The charging resistors 2| and 22together with one or the other of condensers 23 and ii and the gastriode 20 form what is commonly referred to as a "sweep oscillator. Asweep oscillator" is a relaxation oscillator in which a grid controlledgaseous rectifier is employed to periodically discharge a condenser onwhich a charge has gradually been built up through a charging resistorand in which the flashing voltage of the gaseous rectifier can be variedover wide limits by control of the negative grid bias.

As shown in Figure 1, the grid 28 receives its bias through a biasingresistor is which is connected to a grounded conductor 30. l

The cathode 25 is self-biased through resistor 3| and condenser 32.

a flat voltage output y 3 The anode 28 of triode 28 is connected throughresistors 21, 22 and 2|, conductor 88, resistor 88 g and choke 88 to thecathode I8 of rectifier I2.

Three voltage regulators 88, 81 and 88 of the glow discharge type arepreferably connected in series between the grounded conductor 88 and theconductor 88. fully pointed out, it is extremely important that veryclose voltage and frequency regulation be maintained. For this reason itis desirable to have close voltage regulation of the output of theoscillator circuit.

The output circuit of the gas triode 28 is connected through a resistor88 and a coupling condenser 88 to the control grid 8| of a triple gridamplifier tube 82. A grid biasing resistor '48 is connected between thecontrol grid 8| and the grounded conductor 88. A cathode biasingresistor 84 is connected between the cathode 88 and the groundedconductor 88. The suppressor grid 48 is connected to the cathode 88 inthe usual manner. The screen grid 81 is connected through a resistor 88to conductor 88. The plate or anode 88 is connected through a resistor58 to the conductor 88. 'The screen grid 81 is also connected th ough aresistor 8I' and conductor 82 to cathode 28 of the gas triode 28.

The output circuit of the amplifier 82 is connected through a couplingcondenser 88 and conductors 58 and 88 to the twin grids 88 and 81respectively of a twin triode amplifier electron discharge device 88.The cathode 88 is connected through a biasing resistor 88 to ground andthe grids 88 and 81 are connected through a biasing resistor 8| toground. The twin anodes 82 and 88 are connected through resistors and 88respectively and then in common through resistors 88, 81 and 88 andchoke 88 to the oathode l8 of the rectifier I2. 8

The twin triode amplifier 88 is connected through coupling resistors 88and 18 and a coupling condenser H to one control grid 12 of a twintriode amplifier 18. This control grid 12 receives its bias through abiasing resistor 18 which connects grid 12 to ground. The cathode 18 ofthe amplifier 18 receives its bias through a biasing resistor 18'. Theanode 18 which is associated with the control grid 12 is connectedthrough a coupling condenser 11 to the control grid 18 of a beam poweramplifier 18. Anode 18 of amplifier 18 obtains its bias through resistor88 and resistors 81, 88 and choke 88 from rectifier I2.

The second anode 8| and second grid 82 of the amplifier tube 18 areconnected in a conventional push-pull arrangement. More specifically,the anode 8i is connected through a coupling condenser 88 to the controlgrid 88 of a beam power amplifier 88. The control grid 82 is connectedthrough a resistor 88 to the control grid 18 of the beam power amplifier18. It will be noted that the control grid 82 receives its bias througha biasing resistor 81 while the anode 8| receives its bias through aresistor 88, resistors 81 and 88 and choke 88 from rectifier I2. Grid 88of amplifier 88 is connected through a biasing resistor 88 to ground. Itwill also be noted that control grid 18 of amplifier 18 is connectedthrough resistors 88 and 81 to ground.

The cathode 88 of amplifier 88 and the cathode 8I of amplifier 18 areconnected through a biasing resistor 82 to ground. The amplifier tubes18 and 85 each have a screen grid 82 and 88 respectively which areconnected through a conductor 88 and resistor 88 and choke to thecathode circuit of rectifier I2.

As will presently be more 7 From the above description it will beapparent that the twin triode amplifier 18 acts as a driver for the beampower amplifiers 18 and 88. The anodes 88 and 88 of the amplifiers 18and 88 respectively are connected to opposite ends of the primary 81 ofa transformer 88. The midpoint of the primary 81 is connected through aconductor 88 and choke 88 to the cathode circuit of the rectifier I2, bywhich the anodes 88 and 88 receive their necessary positive bias.

The transformer 88 is provided with a secondary 88 whichis connected toan inductance bridge in a manner now to be described. The inductancebridge includes a detector coil I88, a group of resistance elements I8I,I82, I88 and IM, and four dry rectifiers I88, I88, I81 and I88. Therectifiers I88 to I88 are connected in the form of a complete loop byconductors I88, Ill. III and H2. The rectifiers, as diagrammaticallyshown in Figure l, are connected so as to permit current to flow in onlyone direction around the loop. One end of the secondary winding 88 oftransformer 88 is connected through a conductor II8 to the lower end ofdetector coil I88 and also to the lower end of resistance element IN.This conductor III is also grounded. The opposite end of the detectorcoil I88 is connected through a conductor II8 to conductor II2 of therectifier loop. The opposite end of the series connected resistanceelements I8I, I82, I88, and I88 is connected through a conductor III toconductor II8 of the rectifier loop. It will be noted that this isdirectly across the rectifier loop from the point where the detectorcoil I88 is connected. Resistance elements [8| and I82 are preferablymade variable to permit convenient adjustment, and resistance elementsI88 and I88 are arranged to be shorted out through short clrcuitingswitches I I8 and I I1 if desired.

The other end of the secondary winding 88 of transformer 88 is connectedthrough a conductor I I8 and condensers II8 and I28 to conductors I88and III respectively of the rectifier loop.

A milli-ammeter III is connected acIOBs the rectifier bridge betweenconductors I88 and III.

As is further illustrated in Figures 2, 3 and 4 of the drawings, thedetector coil I88 is in the form of a solenoid wound on a powdered ironcore I22.

I have discovered that the wall thickness of a metal valve or otherferro-magnetic structure may be accurately measured by bringing thedetector coil adjacent one side of the wall, and then determining thedifference in the flow of current through the detector coils broughtabout by the change in the inductive value of the coil. It is believedthat this change in inductive value is due to the introduction of amaterial in the magnetic field of the coil and also to the extent ofeddy current penetration in the material. It is further believed thatthe change in inductance in a detector coil is a function of thefrequency of alternation of the current fiowing therein, thepermeability of the material against which the coil is positioned. theresistivity of the material and the thickness of the material (it beingassumed that the material extends out beyond the side of the coil in alldirections).

When tests are to be made on a number of items, all of which are made ofthe same material, it may be assumed that the permeability and theresistivity are constant. If the measuring apparatus is then set foroperation at a given frequency it will readily be determined that theonly variable remaining is the thickness dimension of the material. Itwill thus be apparent indicated at I 26'.

that the meter I2I, as shown in Figure 1 of the drawings, may becalibrated directly in inches when the oscillator is set for somepredetermined frequency.

It has been found in practice that when the thickness of the wall of thehead I23 and the skirt I24 of an aircraft valve I is to be measured thatthe order of magnitude of the frequency selected should be in theneighborhood of 5000 cycles per second. The valves I25, as shown inFigure 2, is partly filled with sodium, as Since the presence of sodiumbehind the wall whose thickness is to be measured will cause adifference in the reading on the meter I2I of the test instrument shownin Figure 1, it is important that the sodium'be moved in the valve to aposition directly behind the wall whose thickness is being measured.This may be conveniently done in any suitable manner, such for example,as vibrating the valve while under a raised temperature, as in themanner described in my copending application entitled "Valve vibratorand heater, U. S. Serial No. 491,665, filed June 21, 1943, now patentNo. 2,400,158, issued -May 14, 1946, and assigned to the same assigneeas the present invention.

The meter I2I of the measuring apparatus shown in Figure 1 is calibratedempirically by noting the readings on the meter for various known wallthicknesses of valves of similar material.

If the skirt I24 of the valve I25 is to be measured, the sodium must bebumped into the skirt part of the valve so that the skirt is entirelybacked up by sodium. Since most valves which contain sodium haveapproximately 75% of the internal space filled with sodium, the sodiummay measuring device as that indicated in Figure 1 may be employed tomeasure resistivity; Under such circumstances, an appropriate scale iscalibrated empirically from known samples for the meter I2I and for thefrequency which is used.

be conveniently bumped behind the skirt by turning the valve upside downfrom its position as.

shown in Figure 2.

I have further found in practice that the sodium content of the valvemay be determined by selecting a much lower frequency and empiricallycalibrating a suitablescale on the meter. When wall thickness of a valveis to be determined, it is believed that a frequency should be selectedin which the eddy current penetration does not extend entirely throughthe wall. That is to say, the eddy current penetration should be lessthan the wall thickness. According to Steinmetz, eddy currentpenetration is given by the following formula: D 225o I uf Where:

D=depth of penetration in inches P=resistivity in ohm-inchesu=permeability v f=frequency in cycles per second When testing for thesodium content of a valve, however, a frequency having the order ofmagnitude in the neighborhood of 180 cycles per second should beselected. The depth of eddy current penetration at this frequencyextends well into the sodium content of the valve. It has been foundthat when the'meter scale is calibrated empirically on the meter I2Ifrom valves having various .amounts of sodiuintherein, that a true andaccurate indication will be given on the mtter I2I of the sodium contentof the valve when the oscillator is set for a relatively low frequencyoutput. This, of course, may be done by properly selecting the condenservalue and charging resistor value of these respective elements which areassociated with the gas triode 20.

Similarly, it will be apparent-that when the thickness of a piece ofmaterial is known andwhere the resistivity of the material is known. themeasuring device of Figure 1 may be employed to measure permeability. Insuch a case an appropriate meter scale is calibrated for the meter I forsome particular frequency and thickness of material.

It will still further be apparent to those skilled in the art that themeasuring device of the present invention is particularly adaptable andsuit-' able for measuring the thickness of a plate or wall member 225which isbacked up by a second and different type of material 226 asdiagrammatically illustrated in Figure 4 which prevents the oppositeside of thematerial being measured from being accessible forconventional types of measuring devices.

A modified form of circuit arrangement for a measuring. device whichwill keep direct current out of the exploring coil or detector coil isdiagrammatically represented in Figure 5 of the drawings. For simplicityof illustration, only three element electron discharge devices have beenshown in this drawing, although it will of course be understood that inpractice multi-electrode types of tubes will generally be foundpreferable.

The circuit as shown in Figure 5 includes a power transformer I22 havinga primary coil I23 arranged to be connected to a suitable source ofalternating current such as the conventional 110 volt 60 cyclealternating current generally provided by public utility companiesthroughout the country. The power transformer I22 is provided with threesecondary windings I24, I26 and I26. The secondary winding I24 providesa low voltage alternating current for heating the filaments of thevarious electron discharge devices of the oscillator and amplifiercircuit pres ently to be described. The winding I26 is the cathodewinding for the full wave rectifier tube I21. The winding I25 has itsopposite ends connected to the two anodes I28 and I29 of the full waverectifier I21. The midpoint I30 of the winding I25 is grounded and oneside of wind-' ing I26 is connected through a choke I3I and bypassed toground by a filter condenser I33. This furnishes positiv plate voltagefor operation of the various electron discharge devices. The fil tercondenser I33 is connected from the midpoint I30 of winding I25 to oneside of the fllae ment winding I26 in order to keep alternating currentout of the various plate circuits of the electron discharge devicesforming the oscillator and amplifier.

a resistor I to the anode I52 of the gas triode I81. The other side oreach of the condensers I88 to I82 is provided with taps I58 to I51 whichare arranged to be selectively engaged by a movable contact element I58which is connected to ground. The contact element I58 is connected to abank of condensers which gives the required basic rrequencies in steps.The control I58, which is a potentiometer, gives gradual frequencycontrol steps. This combination of controls I55 and I58 furnishes avariable means of frequency control covering the entire audio range. Thegrid I58 oi the gas triode I81 is connected through a biasing resistorI55 to ground. The cathode I5I is connected through a self biasingresistor I88 to ground. A by-pass condenser I58 is connected around thebiasing resistor I82 in a conventional manner. A plurality of voltageregulator tubes I88, I55 and I85 are connected in series between themovable contact arm I55 and ground, in order to provide stable andaccurate voltage regulation or the oscillator circuit. The output of theoscillator is connected through a resistor I51 and coupling condenserI58 to the grid I58 of an amplifier I15. The anode "I of the amplifier Iis connected through resistors I12 and I18 to the high potential biasingconductor I82 of the rectifier circuit. The movable contact element I55of the charging resistor I88 is also connected to the conductor I18which extends between the resistor I12 and the resistor I18.

The cathode I15 of the amplifier I15 is connected through a biasingresistor I15 to ground. Filter condenser I11 is preferably shuntedacross the biasing resistor I15. By-pass condenser I18 is alsopreferably provided between movable contact element I55 and ground, orin other words, is connected in parallel around the three voltageregulators I58, I55 and I55.

Two additional stages of amplification are provided by amplifiers I18and I85. The anode "I of the amplifier I18 is connected through acoupling condenser ill to the grid I82 of the amplifier I18. The cathodeI88 of the amplifier I18 is provided with a self biasing resistor I88and a conventional by-pass condenser I85. The grid is provided with abiasing resistor I88 as shown. The anode I81 receives its bias. throughresistors I88 and I88 which are connected to the high potential biasingconductor I82. Filter condensers I88 and I8I are preferably provided asshown.

The anode I81 is connected through a coupling condenser I82 to the gridI88 of the amplifier I88. Grid I88 is provided with a biasing resistor I88 connecting the grid to ground. The cathode I 88 is provided with aself biasing resistor I88 and a conventional by-pass condenser I81. Theanode I88 is connected through resisgor I88 to the high potentialbiasing conductor I8 The output of the amplifier I88 is connectedthrough a coupling condenser 258 to grids I and 252 of electrondischarge devices 258 and 888. The two grids "I and 252 which areconnected together are provided with a common biasing resistor 288 whichis connected to ground. The filaments 288 and 251 or the tubes 288 and258 form directly heated cathodes. These filaments 255 and 251 areconnected across windings 258 and 258 which are energized from anysuitable source of alternating current. A meter in the form of amilli-ammeter 2I5 is connected between the midpoints 2| I, 2I2 of thewindings 258 and 858. These midpoints 2H and 2I2 are 8 also connectedthrough biasing resistors 2I8 and 2I8 rupectively to ground. Two by passcondensers 2I8 and 2I8 are provided around the biasing resistors 2I8 and2I8.

The detector coil of this embodiment: of the invention is given the samereference character I58 as was given the detector coil I88 in thepreferred embodiment of the invention as illustrated in Figures 1 to 4of the drawings. in order to avoid confusion. The detector coil I55 isconnected in series with a condenser 2" across a choke coil 2I8. Thiscondenser 2" stops the dew of direct current through the detector coilI55 and allows the A. C. voltage irom the amplifier only to be impressedon the coil I58. The choke 2I8 may be replaced with a suitable step-downtransformer. This permits use of a much smaller detector coil which isconnected to the secondary of this transformer. The choke coil 2I8 isconnected at one end to the anode 2I8 of the tube 258 while it isconnected at its other end to a movable contact arm 228 of a variableresistor 22I connected to the anode 222 of the tube 258. The movablecontact 228 is also connected to ground through the conductor 228.

In this particular form of the invention the current flowing through thechoke 2 I8 and detector circuit to the amplifyin tube 258 is balancedagainst the current flowing through the resistor Hi to the otheramplifying tube 258, the condition of balance being indicated by meter2I5. When the inductive value of the detector coil I55 is changed bybringing it into proximity of the wall whose thickness is to bemeasured, the balance on the meter 2I8 is changed and the variation inthe positon of the needle 228 is a function of the thickness of thematerial against which the detector coil I is placed. A meter scale isprovided for the meter 2I8 which is calibrated empirically from knownvalues. The advantage of the operation as shown in Figure 5 lies in thefact that no direct current flows through the detector coil I58. It hasbeen found that under many circumstances more accurate readings ofthickness or of other values to be determined may be had when directcurrent is kept out or the detector coil.

While I have shown and described certain particular embodiments of myinvention, it will, of course, be understood that I do not wish to belimited thereto, since many modifications may be made, and I. therefore,contemplate by the ap pended claims to cover all such modifications asgall within the true spirit and scope of my inven- I claim as myinvention:

1. The method of measuring the thickness of a para-magnetic wallenclosing a cavity containing sodium which includes causing the sodiumto be disposed directly behind the wall to be measured, then bringingthe wall into the concentrated region of a magneflc field produced by acoil carrying alternating current, adjusting the frequency ofalternation to a sufiiciently great value so that eddy currents producedby the alternating field will not penetrate through the wall into thesodium, and detecting the change in the apparent inductance of the coil.

2. The method of measuring the head wall thickness of a hollow steelvalve partially filled with sodium which includes bumping substantiallyall of the sodium into the head directly be hind the head wall,positioning the head wall opposite the end of and in close proximity toa coil carrying alternating current whose frequency of alteration isselected suiiiciently high that the depth of eddy current penetration isless than the wall thickness, and detecting the change in the apparentinductance of the coil caused by bringing the head wall into themagnetic field of the coil.

3. The method of measuring the sodium content of a hollow valvepartially filled with sodium which includes bumping substantially all ofthe sodium into the head of the valve directly behind the head wall,positioning the head wall against the end of a coil carrying alternatingcurrent, adjusting the frequency of alternation to a value suflicientlylow so penetration is greater than the wall thickness, and measuring thechange in the apparent impedance of the coil.

4. The method of measuring the sodium content of a hollow valvepartially filled with sodium which includes moving substantially all ofthe sodium into one portion of the valve directly behind one wall of thevalve, positioning said wall opposite the end of and in close proximityto a coil carrying alternating current, adjusting the frequency ofalternation to the order of magnitude of several hundred cycles persecond, and measuring the change in the apparent impedance of the coil.

5. The method of analyzing a ferromagnetic container partially filledwith a nonmagnetic, electrically-conducting material, which comthat thedepth of eddy current thickness of the wall respectively, by detectingthe effect on the apprises tilting said container to a position so thata portion thereof fully encloses atleast part of said non-magneticmaterial and.positioning an inductance coil energized by alternatingcurrent in contacting relationship with said container portion anddetecting the apparent impedance change of said coil caused thereby.

8. The method of measuring the thickness of a ferromagnetic containerpartially filled with'a non-magnetic, electrically-conducting material,

which comprises positioning said container so that a port on thereofencloses said non-magnetic material, then placing an inductance coilenergized by alternating current in close proximity to said containerportion and adjusting the frequency of said alternating current so thatthe depth of eddy current penetration will be less than the thickness ofsaid container and detectingl the effect on the apparent impedance ofsaid co 7. The method of measuring the thickness of a hollow steelpoppet valve partially filled with sodium which comprises positioningsaid valve so that sodium is contained within the head portion of thevalve, applying an inductance coil energized by alternating current tothe head portion of said valve, adjusting the frequency of saidalternating current to the neighborhood 01-5000 cycles per second sothat the eddy current penetration will be less than the thickness of thewall 01' said head portion, and detecting the eflect on the apparentimpedance of said coil.

8. The method of selectively measuring the thickness of a ferromagneticcontainer and the amount of non-magnetic. electrically conductingmaterial enclosed thereby which comprises tilting said container to sucha position that a portion thereof fully contains said non-magneticmaterial, applying an inductance coil energized by alternating currentin contacting relationship with said container portion and selectivelyadlusting the frequency of said alternating current to relatively highand low values, to measure the time adjusting parent impedance of saidcoil, said high value of frequency being sufllclent to prevent eddycurrent penetration through said wall and said low value of frequencybeing selected to produce eddy current penetration of said wall andcontents.

9. The method of selectively measuring the thickness of a ferromagneticcontainer and the amount of non-magnetic, electrically conductingmaterial enclosed thereby, which comprises tilting said container tosuch a position that a portion thereof fully contains said non-magneticmaterial, applying an inductance coil energized by alternating currentin contacting relationship with said container portion and selectivelyad justing the frequency of said alternating current to the neighborhoodof 5000 cycles per second so as to measure the thickness of said wallportion devoid of eddy current penetration through the entire thicknessof said wall portion, and at a different time adjusting the frequency tothe neighborhood of cycles per second to measure said non-conductingmaterial contents, and measuring the changes in apparent impedance ofsaid inductance coil at both frequencies caused by the presence of saidcontaine 10. The method of discriminating between the impedance edectscaused contents of a ferromagnetic valve enclosing sodium, whichcomprises completely backing up a wall by sodium and applying wallportion of said an inductance coil energized by alternating current incontacting relationship with said wall portion of said valve, andselectively adjusting the frequency of said alternating .current atvalues of several thousand cycles per second so as to confine eddycurrent penetration to less than the thickness of said wall formeasuring the wall thickness of said wall, and at a different thefrequency to values of about one hundred cycles per second to insuresubstantial eddy current penetration through the sodium for analyzingthe sodium content, and measuring the apparent impedance change of saidcoil at both frequencies caused by application of the coil to said wallportion. v REX DE ORE McDILL.

REFERENCES CITED The following references are of record in the flle'ofthis patent:

UNITED STATES PATENTS Number Name Date 1,807,411 Imes May 26, 19311,952,185 Smith Mar. 27, 1934 41 Rhodes et al. June 26, 1934 2,008,141Snelllng July 16, 1935 2,020,067 Keinath Nov. 5, 1935 2,111,210 EbelMar. 15, 1938 2,116,119 Loewenstein May 3, 1938 2,226,275 Abbott et al.Dec. 24, 1940 2,337,231 Cloud Dec. 21, 1943 FOREIGN PATENTS NumberCountry 1 Date 388,006 England Sept. 25. 1940 OTHER REFERENCES Kuehni.

General Electric Review, September 1942, pages 533-630. v

by wall thickness and

